1. Field of the Invention
The present invention relates to a method selective of vapor phase etching for fabricating semiconductor devices, and particularly to such a method comprising a step of removing a silicon oxide film formed on a semiconductor substrate.
2. Description of the Related Art
There will be explained hereinafter steps of removing a silicon oxide film formed on a semiconductor substrate according to conventional semiconductor device fabrication methods. A similar conventional technique is proposed in the gazette of Japanese Patent Laid Open (Kokai) No. 5(1993)-74817.
First, with reference to FIG. 4, a method of fabricating a semiconductor device having a GaAs MESFET (metal semiconductor field effect transistor) construction will be described as a first conventional example.
FIG. 4(a) is a cross sectional diagram depicting a semiconductor substrate with a silicon oxide film 6 formed on its surface. The semiconductor substrate 1 comprises a semi-insulating GaAs substrate 1, an undoped GaAs buffer layer 2, a GaAs channel layer 3 which is doped with Si impurities, and a GaAs cap layer 5 which is doped with Si impurities and is formed only at a region thereof on which a source and a drain are to be formed.
The silicon oxide film 6 is removed over a portion of the semiconductor substrate at which a gate is to be formed, and a Schottky metal film 7, a tungsten silicide (WSi) film in this example, is deposited by means of a conventional sputtering method. On this WSi film, a metal film 8 for further decreasing a gate resistance, a gold (Au) film in this example, is formed by means of a conventional sputtering method. Next, by conventional photolithography and etching technique, such as ion-milling or dry-etching using resist films as masks, the metal films comprised of the WSi film 7 and the Au film 8 are removed except for a region on which a gate electrode is to be formed. After that, the resist film is removed, to obtain a gate electrode as shown in FIG. 4(b).
At this time, a thick silicon oxide film 6 having a dielectric constant of approximately 4 exists adjacent to the gate electrode. Accordingly a capacitance associated with the gate electrode will be so large that the high frequency characteristics of the FET is severely deteriorated. Therefore, as shown in FIG. 4(c), this thick silicon oxide film 6 is removed by means of wet etching using an etching liquid containing hydrofluoric acids. After that, as shown in FIG. 4(d), a thin silicon oxide film 10 is formed as a passivation film.
The reasons that wet etching by hydrofluoric acids is used for removing the silicon oxide film is: that the etching should be isotropic in order to etch under the eave of a gate metal, that dry etching induces crystallographic damage to the semiconductor, and that wet etching is advantageous from the view point of production cost.
Lastly, as shown in FIG. 4(e), ohmic electrodes 11, e.g. AuGeNi in this example, corresponding to a source electrode and a drain electrode, are formed by steps of forming an opening on the silicon oxide passivation film 10, depositing, lifting-off, and annealing a metal film for the ohmic electrode, and so forth, thereby completing the MESFET structure fabrication.
Another conventional method of producing a semiconductor device having an AlGaAs/GaAs HJFET (hetero junction field effect transistor) construction will be described with reference to FIG. 5.
In FIG. 5(a), the semiconductor substrate 1 comprises a semi-insulating GaAs substrate 1, an undoped GaAs buffer layer 2, a GaAs channel layer 3 which is not doped, an Al.sub.0.2 Gao.sub.0.8 As layer 4 which is made into an election supplying layer, and an GaAs cap layer 5 which is doped with impurities of high concentration and is left over only at a region thereof on which a source and a drain are to be formed.
First, the silicon oxide film b is partially removed over a portion of the semiconductor substrate at which a gate is to be formed. Subsequently, a Schottky metal film 9, e.g. a tungsten (W) film in this example, is deposited by a conventional sputtering method. Next, by means of the usual photolithography and dry etching method using a photoresist as a mask, the metal film 9 is selectively etched away to form a gate electrode. After that, the photoresist is removed forming a gate electrode as shown in FIG. 5(b).
Hereafter, similar to the first conventional example, the silicon oxide film 6 is removed by wet etching using the hydrofluoric acid based etching liquid (FIG. 5(c)). Next, a passivation silicon oxide film 10 is formed over the entire surface as shown in FIG. 5(d). Finally, ohmic electrodes 11 corresponding to a drain electrode and a source electrode are formed (FIG. 5(e)), thereby completing a semiconductor device having an HJFET structure.
In such conventional semiconductor device fabrication methods, heat resistant refractory metal silicides such as tungsten silicide (WSi) 7 are frequently used as a Schottky metal, due to reliability considerations. In such cases, there is a problem that when the silicon oxide film 6 is wet etched by the hydrofluoric acid based etching liquid, a lower side wall portion of the WSi film 7 is partially dissolved as shown in FIG. 4(c). The reason for this phenomena is understood to be that when the refractory metal silicide film 7 is formed within the opening formed on the silicon oxide film 6 by sputtering, a portion of the silicide film 7 adjacent to the silicon oxide film 6 is changed in chemical composition and physical structure, so that a region that does not have resistance to the hydrofluoric acid based etching liquid is formed. When this dissolution of the refractory metal silicide occurs, the shape of the gate electrode is locally deformed, so that the distribution of the electric field and the current density becomes non-uniform, resulting in the deterioration of the electrical characteristics and the reliability of the FET.
In the second conventional example, where the layer just below the silicon oxide layer 6 is the AlGaAs 4 layer there is a problem that a portion of the AlGaAs 4 layer located in the vicinity of the gate electrode metal is dissolved during wet etching with the hydrofluoric acid based liquid due to an electrochemical reaction. In this case, the shape of the surface of the semiconductor is changed and the depletion layer formed beneath is deformed, which brings a decrease in the saturation current.